Recently, there has been an increasing interest in energy storage technologies. As batteries have been widely used as energy sources in the fields of mobile phones, camcorders, notebook computers and even electric cars, research and development has been increasingly made on the batteries. Among the batteries, electrochemical devices, in particular, rechargeable secondary batteries are the center of attention. Recent trends of study move toward new designs of electrodes and batteries to improve capacity density and specific energy.
Among currently available secondary batteries, lithium secondary batteries that were developed in early 1990's have a higher operating voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. These characteristics of the lithium secondary batteries afford advantages. However, the lithium secondary batteries have disadvantages such as a complicate manufacturing process and safety-related problems caused by use of an organic electrolyte, for example, firing, explosion, and the like. Lithium-ion polymer batteries are considered as more suitable next-generation batteries than lithium ion batteries. However, lithium-ion polymer batteries have a relatively lower battery capacity than lithium ion batteries, and have an insufficient discharging capacity at low temperature. Accordingly, there is an urgent need to solve these disadvantages of the lithium-ion polymer batteries.
A variety of electrochemical devices have been produced from many companies, and each exhibits different safety characteristics. Thus, it is important to evaluate and ensure safety of the electrochemical devices. First of all, electrochemical devices should not cause any damage to users due to malfunction. Taking this into account, Safety Regulations strictly prohibit safety-related accidents of electrochemical devices such as firing or smoke emission. According to the safety characteristics of electrochemical devices, explosion may occur when an electrochemical device is overheated and subject to thermal runaway, and when a separator is punctured. In particular, when an electrochemical device is at a temperature of 100° C. or above, a polyolefin-based porous substrate that is commonly used as a separator of the electrochemical device shows a significant thermal shrinking behavior due to its material characteristics and manufacturing characteristics such as elongation, so that a short circuit may occur between a cathode and an anode.
In order to solve the above safety-related problems of the electrochemical device, a separator has been suggested in which an organic-inorganic composite porous coating layer made from a mixture of a binder polymer and inorganic particles is formed on at least one surface of a porous substrate with a plurality of pores. For example, Korean Patent Laid-open No. 2007-0019958 discloses a separator in which a porous coating layer made from a mixture of a binder polymer and inorganic particles is formed on the surface of a porous substrate.
In a separator with the organic-inorganic composite porous coating layer, the inorganic particles in the porous coating layer formed on the porous substrate serve as a spacer that keeps a physical shape of the porous coating layer, so the inorganic particles may restrain thermal shrinkage of the porous substrate when an electrochemical device is overheated, and accordingly, prevent a short circuit between a cathode and an anode from occurring when thermal runaway occurs at the electrochemical device.
To show the above-mentioned functions of the organic-inorganic composite porous coating layer formed on the porous substrate, the inorganic particles should be sufficiently included in at least a predetermined content. However, the higher content of the inorganic particles, the relatively lower content of the binder polymer, and as a result, bondability of the separator to electrodes is reduced and the inorganic particles are apt to detach from the porous coating layer due to contact with the external environment or stresses occurring during an assembling process of an electrochemical device such as a winding process. The reduction in bondability to electrodes results in reduction in performance of the electrochemical device, and the detached inorganic particles act as a local defect of the electrochemical device, thereby giving a bad influence on safety of the electrochemical device.